Gets the current rectangular clipping region. The
clipping width and height are set to -1 if there
is no current clipping region.
(send a-wx:dc get-clipping-region xywidthheight)->void x : boxed inexact real y : boxed inexact real width : boxed inexact real height : boxed inexact real
The x box is set to the top-left location of the clipping
region (in DC coordinates), etc.